1. Field of Invention
Embodiments of this invention relate to polymer electronic devices and methods of producing polymer electronic devices, and more particularly to all-solution processes and polymer electronic devices made by all-solution processes.
2. Discussion of Related Art
The contents of all references referred to herein, including articles, published patent applications and patents are hereby incorporated by reference.
Electronic devices based on organic materials (small molecules and polymers) have attracted broad interest. Such devices include organic light emitting devices (OLEDs) (Tang, C. W.; VanSlyke, S. A.; Appl. Phys. Lett. 1987, 51, 913), organic photovoltaic cells (OPVs) (Tang, C. W. Appl. Phys. Lett. 1986, 48, 183), transistors (Bao, Z.; Lovinger, A. J.; Dodabalapur, A. Appl. Phys. Lett. 1996, 69, 3066), bistable devices and memory devices (Ma, L. P.; Liu, J.; Yang, Y. Appl. Phys. Lett. 2002, 80, 2997), etc. Some of the most salient attributes of polymer electronics is that they can be very low-cost, flexible, operate with low-energy consumption, can be produced with high-throughput processing, and can be versatile for a range of applications (Forrest, S. R. Nature 2004, 428, 911). To achieve low cost production, solution processing is highly desirable.
Solar cells, also known as photovoltaic (PV) cells or devices, generate electrical power from incident light. The term “light” is used broadly herein to refer to electromagnetic radiation which may include visible, ultraviolet and infrared light.
Traditionally, PV cells have been constructed of a number of inorganic semiconductors, e.g., crystalline, polycrystalline and amorphous silicon, gallium arsenide, cadmium telluride and others. More recently, PV cells have been constructed using organic materials.
Solar cells are characterized by the efficiency with which they can convert incident solar power to useful electric power. Devices utilizing crystalline or amorphous silicon dominate commercial applications, and some have achieved efficiencies of 23% or greater. However, efficient crystalline-based devices, especially of large surface area, are difficult and expensive to produce due to the problems inherent in producing large crystals without significant efficiency-degrading defects. On the other hand, high efficiency amorphous silicon devices still suffer from problems with stability. Present commercially available amorphous silicon cells have stabilized efficiencies between 4 and 8%. More recent efforts have focused on the use of organic photovoltaic cells to achieve acceptable photovoltaic conversion efficiencies with economical production costs as well as other possible advantageous properties.
PV devices produce a photo-generated voltage when they are connected across a load and are irradiated by light. When irradiated without any external electronic load, a PV device generates its maximum possible voltage, V open-circuit, or VOC. If a PV device is irradiated with its electrical contacts shorted, a maximum short-circuit current, or ISC, is produced. (Current is conventionally referred to as “I” or “J”.) When actually used to generate power, a PV device is connected to a finite resistive load in which the power output is given by the product of the current and voltage, I×V. The maximum total power generated by a PV device is inherently incapable of exceeding the product ISC×VOC. When the load value is optimized for maximum power extraction, the current and voltage have values, Imax and Vmax, respectively. A figure of merit for solar cells is the fill factor, ff (or FF), defined as:
  ff  =                    I        max            ⁢              V        max                            I        SC            ⁢              V        OC            where ff is always less than 1, as ISC and VOC are never achieved simultaneously in actual use. Nonetheless, as ff approaches 1, the device is more efficient.
When electromagnetic radiation of an appropriate energy is incident upon a semiconductive organic material, for example, an organic molecular crystal (OMC) material, or a polymer, a photon can be absorbed to produce an excited molecular state. This energy absorption is associated with the promotion of an electron from a bound state in the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO), or equivalently, the promotion of a hole from the LUMO to the HOMO. In organic thin-film photoconductors, the generated excited state is believed to be an exciton, i.e., an electron-hole pair in a bound state which is transported as a quasi-particle. The excitons can have an appreciable life-time before recombination. To produce a photocurrent the electron-hole pair must become separated, for example at a donor-acceptor interface between two dissimilar contacting organic thin films. The interface of these two materials is called a photovoltaic heterojunction If the charges do not separate, they can recombine with each other (known as quenching) either radiatively, by the emission of light of a lower energy than the incident light, or non-radiatively, by the production of heat. Either of these outcomes is undesirable in a PV device. In traditional semiconductor theory, materials for forming PV heterojunctions have been denoted as generally being of either n (donor) type or p (acceptor) type. Here n-type denotes that the majority carrier type is the electron. This could be viewed as the material having many electrons in relatively free energy states. The p-type denotes that the majority carrier type is the hole. Such material has many holes in relatively free energy states. The type of the background majority carrier concentration depends primarily on unintentional doping by defects or impurities. The type and concentration of impurities determine the value of the Fermi energy, or level, within the gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO), called the HOMO-LUMO gap. The Fermi energy characterizes the statistical occupation of molecular quantum energy states denoted by the value of energy for which the probability of occupation is equal to ½. A Fermi energy near the LUMO energy indicates that electrons are the predominant carrier. A Fermi energy near the HOMO energy indicates that holes are the predominant carrier. Accordingly, the Fermi energy is a primary characterizing property of traditional semiconductors and the PV heterojunction has traditionally been the p-n interface.
A significant property in organic semiconductors is carrier mobility. Mobility measures the ease with which a charge carrier can move through a conducting material in response to an electric field. As opposed to free carrier concentrations, carrier mobility is determined in large part by intrinsic properties of the organic material such as crystal symmetry and periodicity. Appropriate symmetry and periodicity can produce higher quantum wavefunction overlap of HOMO levels producing higher hole mobility, or similarly, higher overlap of LUMO levels to produce higher electron mobility. Moreover, the donor or acceptor nature of an organic semiconductor may be at odds with the higher carrier mobility. The result is that device configuration predictions from donor/acceptor criteria may not be borne out by actual device performance. Due to these electronic properties of organic materials, the nomenclature of “hole-transporting-layer” (HTL) or “electron-transporting-layer” (ETL) is often used rather than designating them as “p-type” or “acceptor-type” and “n-type” or “donor-type”. In this designation scheme, an ETL will be preferentially electron conducting and an HTL will be preferentially hole transporting.
Organic PV cells have many potential advantages when compared to traditional silicon-based devices. Organic PV cells are light weight, economical with respect to the materials used, and can be deposited on low cost substrates, such as flexible plastic foils. (See, for example, J. J. M. Halls et al. Efficient photodiodes from interpenetrating polymer networks. Nature 376, 498 (1995); N. S. Sariciftci, L. Smilowitz, A. J. Heeger, and F. Wudl, Photoinduced Electron Transfer from a Conducting Polymer to Buckminsterfullerene. Science 258, 1474 (1992); C. J. Brabec, J. A. Hauch, P. Schilinsky, and C. Waldauf, Production aspects of organic photovoltaics and their impact on the commercialization of devices. MRS bulletin, 30, 50 (2005); René A. J. Janssen, Jan C. Hummelen, and N. Serdar Sariciftci, Polymer-fullerene bulk heterojunction solar cells. MRS bulletin, 33, 50 (2005); C. W. Tang, Two-layer organic photovoltaic cell. Appl. Phys. Lett. 48, 183 (1986); P. Peumans, A. Yakimov, and S. R. Forrest, Small molecular weight organic thin-film photodetectors and solar cells. J. Appl. Phys. 93, 3693 (2003); and G. Li, et al. High-efficiency solution processable polymer photovoltaic cells by self-organization of polymer blends. Nat. Mater. 4, 864 (2005).) Polymer solar cells have a typical structure of anode/polymer layer/cathode. For such devices, thermal evaporation processing under ultra-high vacuum is conventionally used for the deposition of the final cathode. This conventional process uses expensive and time-consuming vacuum deposition for the metal contacts. Small molecule based organic solar cells have a particular disadvantage in that they require critical control of the thickness of the multilayer device structure over a large area. In addition, low work function metals such as calcium are typically used in order to improve the performance. The degradation of the reactive metal cathode and metal/polymer interface inevitably leads to decreased device performance and sophisticated encapsulation schemes are required to prevent reaction with moisture and oxygen in the air. Consequently, there remains a need for improved organic electronic devices.